The inventive concept relates to optical waveguides and, more particularly, to optical waveguides based on surface plasmons.
Surface plasmons are charge density oscillations confined at an interface between two materials of which dielectric constants have their respective real parts having signs opposite to each other. The surface plasmons may be easily formed at an interface between a metal having a dielectric constant with a negative real part and a dielectric having a dielectric constant with a positive real part. The surface plasmons may be excited by an electron beam or a light wave.
An electromagnetic wave combined with the surface plasmons is defined as surface plasmon polaritons (SPPs). Since a wave vector of the electromagnetic wave combined with the surface plasmons is greater than a wave vector of an electromagnetic wave in a surrounding medium, the SPPs are confined in a metal surface and then propagate along the metal surface. The SPPs have a transverse magnetic (TM) polarization with respect to the metal surface. An electromagnetic field intensity of the SPPs has the maximum value at the interface between the metal and the dielectric and is exponentially decayed as a distance from the interface of the metal and the dielectric increases. Thus, the interface of the metal and the dielectric may be used as a plane optical waveguide having a confinement condition along a direction perpendicular to the interface.
A SPP waveguide may generally have a very high propagation loss. Thus, it may be difficult to apply the SPP waveguide to a practical optical circuit with a length over several tens μm or hundreds μm. Meanwhile, if a thickness of a metal layer is equal to or less than a skin depth for an operating wavelength, SPPs guided by top and bottom surfaces of the metal may be combined with each other, such that a long-range SPP (LRSPP) mode with a very low propagation loss may be formed to transmit signals by a long distance of several mm to several tens cm. An electromagnetic field of the LRSPP mode may penetrate more strongly and widely into the dielectric around a metal thin layer than into the inside of the metal, such that the propagation loss of the light may be very low. Additionally, the LRSPPs may have excellent coupling characteristics with an optical fiber. Thus, the LRSPPs may be applied to various areas of optical devices.